This invention relates generally to semiconductor injection lasers and, more particularly, to high-power surface-emitting semiconductor injection lasers.
High-power laser light sources are required for a variety of optical systems, such as optoelectronic logic circuits, fiber optic communication systems, and for optically pumping solid state lasers. Semiconductor injection lasers are particularly well suited as laser light sources for these optical systems because of their small size, reliability, and operation at wavelengths having low transmission and dispersion losses in glass fiber optics.
A semiconductor injection laser is a diode device in which a forward bias voltage is applied across an active layer formed between n-doped and p-doped cladding layers. Excess electrons from the n-doped layer and excess holes from the p-doped layer are injected into the active layer by the bias voltage, where the excess electrons and holes recombine. At low current levels, the electrons recombine with the holes to produce spontaneous emission of photons in all directions. At current levels above the laser threshold value, the excess carrier density becomes high enough to produce an inverted population, yielding a positive gain. Stimulated emission occurs and a monochromatic, highly-directional beam of light is emitted from the active layer. A resonant cavity can be formed in the active layer by cleaved facets at either end of the device, one being a highly-reflective surface and the other being a partially-reflective surface through which the beam emerges. The resonant cavity can also be bounded by etched side surfaces, to prevent emission in the lateral direction, and by the cladding layers, which have indexes of refraction that are lower than the active layer to confine the light to the plane of the active layer.
The power output of a single semiconductor injection laser is rather small and is inadequate for most types of optical systems. Therefore, semiconductor injection lasers are typically combined in large two-dimensional arrays to provide increased power levels. Surface-emitting semiconductor injection lasers are particularly well suited for fabricating large two-dimensional monolithic laser arrays because of their emission of laser light from a top or bottom surface of the device. However, many surface-emitting semiconductor lasers exhibit poor thermal characteristics because of poor thermal conduction between the active layer and the heat sink. Accordingly, there has been a need for a surface-emitting semiconductor injection laser having good thermal conduction properties for operation at high power levels. The present invention is directed to this end.